add unnecessary files/folders to export-ignore

`/tests`, `/phpunit.xml.dist`, and `/.travis.yml` aren't needed when pulling down the package via Composer, so we can add them to the `export-ignore` list to prevent them from being downloaded.

Author: svenluijten

.gitattributes

@@ -3,4 +3,7 @@
 /.gitattributes export-ignore
 /.gitignore export-ignore
 /README.md export-ignore
-/examples export-ignore
+/examples export-ignore
+/tests export-ignore
+/phpunit.xml.dist	export-ignore
+/.travis.yml export-ignore